Recently, a semiconductor device has been designed to have multilayer interconnection for the purpose of improving its operating speed and reducing its size. Further, it is necessary to reduce resistances of lines and electric capacitances between lines in order to increase its operating speed. Accordingly, copper (Cu) having a low resistance is widely used in the lines, and a low dielectric constant material is widely used in an interlayer insulating film provided between the Cu lines to reduce capacitance between the Cu lines.
The interlayer insulating film made of a low dielectric constant material (low-k film) is formed on a surface of a semiconductor wafer by employing a spin on dielectric (SOD) method or a chemical vapor deposition (CVD) method. In the SOD method, a coating solution is supplied to the surface of the semiconductor wafer and the coating solution is spread by rotating the semiconductor wafer, thereby forming the low-k film. In the CVD method, a source gas is supplied to the surface of the semiconductor wafer and products are deposited by decomposition or composition through chemical reactions, thereby forming the low-k film.
In a case in which the low-k film is formed by the SOD method, the heat treatment is performed on the semiconductor wafer after the low-k film has been formed to relieve internal stress in the low-k film and to ensure its mechanical strength. Further, even in a case in which the low-k film is formed by the CVD method, the heat treatment is occasionally required for a certain selected low-k material. The heat treatment is generally performed in a vacuum or nitrogen gas atmosphere. However, it is very difficult to make a complete vacuum or nitrogen gas atmosphere, and impurities such as oxygen can be easily contained in the atmosphere. Accordingly, in these methods, there is a worrisome possibility that the low-k film may be degraded (oxidized) due to oxygen contained in the atmosphere.
Thus, it has been attempted to perform the heat treatment in an atmosphere of a hydrogen gas or ammonia gas widely used as a reactant (reducing) gas (see, for example, Patent Document 1). However, it is difficult to completely prevent degradation of the low-k film with reactivity of a hydrogen gas or ammonia gas.
Patent Document 1: Japanese Laid-open Publication No. 2003-158126